The invention is a method and means for control of traffic on a network with route control. The method and means are based on functions in a traffic management system. The invention includes control of traffic from a first route, which passes a bottleneck, which can be a part of the network, e g a node or a link, with low capacity relative to the demand of traffic through the node or the link,xe2x80x94to at least another alternative route. The invention concerns traffic control of vehicles on a road network as a first hand application. But the method can also be used for other applications, e g traffic control of vehicles on rail networks, air traffic network control and sea traffic network control, and as traffic control of data packets on a communication network. This application is also treated.
Field of the Invention and Description of the Related Art
Traffic volumes are large during rush hours and there are queues growing on the network in and outside large cities. It is difficult to find space for more roads and those is expensive to build. By use of advanced information technology the existing capacity of the road network can be utilized better and thereby larger traffic volumes can be handled with less additions of road capacity.
This is reflected in the large interest, which is devoted ITS, Intelligent Transport Systems, within EU, USA and Japan et al. during the nineties. How the solutions would look like is however unclear, and therefore large amounts of money is invested in research in the domain, and several different ideas are studied.
Traditionally people have tried to solve capacity problems in the road network, by building more roads or by taking actions in those points, where problems appear. If there are long queues on a road upstream an intersection, people are trying to increase the ability to pass through the intersection for the cars on the said road. This is the traditional way of looking on traffic problems. The problems consist of narrow sections in the road network. In those points traffic queues arise, and therefore people consider the solution being limited to an increase of the flow capacity in those points.
With a deeper knowledge of traffic, and traffic network characteristics, the traditional xe2x80x9cpoint-orientedxe2x80x9d way of work appears as superfluous and inadequate. Performed xe2x80x9csolutionsxe2x80x9d may create larger problems than the problem they solve. An example is given below.
It is not unusual with queues on the entrance roads of the large cities during the rush hour. If a queue is arisen at a narrow section, e g at an on-flow link to the entrance road, and the ability to pass is increased in this point by e g adding an extra lane, the increased flow might be trapped in a new narrow section, whereby queues are built up there instead. Queues at the new spot might create larger problems than the queue at the former spot.
There is a need for a more system oriented way of work for solving xe2x80x9cthe ability to passxe2x80x9dxe2x80x94problems in a network.
Route control has traditionally been used for certain events, e g road works on a link, when traffic signs guide the traffic into other links around the link with road works, or e g when a larger accident occurred on a link, and the police is there directing the traffic to other links around the place of the accident. This cannot always be done free from secondary problems. If the roadwork or the accident is on a link with large traffic, the new appointed route might not have enough capacity to carry all the new traffic too, and long queues can arise. In large cities the road network is generally heavily loaded during rush hours, and incidents, which suddenly reduce the capacity of a busy link, might easily cause long queues. Those queues in their turns are blocking traffic also on other routes, why ability to pass would be strongly reduced for large parts of the network.
Traditionally route control has been used on corresponding point oriented ways as described above. When a problem appears in a point in the network, traffic is directed away from that point. Then the problem might be solved in that point, but traffic might cause worse problems on other points in the network.
There is a need for more system-oriented methods for solving traffic problems in a road network.
During the nineties the international investments on information technology for vehicle traffic, ITS, have given rise to some new concepts and ideas, of which some will be commented below.
Route management in the shape of DRG, xe2x80x9cDynamic Route Guidancexe2x80x9d, has been treated in ITS-projects in EU- and USA-research programs. (The inventor has taken part in such an EU-project). In the concept vehicles are equipped with navigation equipment, Neq, and a central system is supplying Neq with travel times for links in the network. Neq then can select xe2x80x9cthe best routexe2x80x9d, (e g the fastest) through the network. There still today is a spread opinion, that traffic thereby would perform in an almost xe2x80x9coptimalxe2x80x9d way with a minimum of queues. Superficially it might seam that DRG gets this function: If Neq selects the xe2x80x9cfastest routexe2x80x9d, the vehicle would avoid places with long queues, and if many (or all) vehicles have got Neqs, the result would be that there never would be any long queues, as the vehicles then would select other routes, and that traffic would be distributed at the network in such a way that alternative routes would take about the same time. The road network would be utilized optimally and the traffic route control would be almost perfect.
Another discussed concept, called xe2x80x9cLisbxe2x80x9d below, (probably from Siemens) has also equipment in the car, which is communicating with a central system. Here the idea is that the driver at start puts in his destination. The central system returns a route, which the driver would follow. At several positions (stations) along the road network, e g at intersections, there are local short-distance communication links for information transfer, where the vehicles identify themselves and when needed obtain an updated route.
An established opinion is that the central system, which knows the positions and destinations of the vehicles, can give each vehicle an optimal route. If many (or all) vehicles take parts in the system and the central system often is updated, the central system has xe2x80x9cfull controlxe2x80x9d of the vehicles and the traffic, and thereby can optimize the traffic on the road network. The road network would be optimally utilized and the traffic route control would be almost perfect.
The present invention is neither DRG- nor Lisb-concept. Those have problems or shortages, which probably don""t appear in a superficial analysis, but will be commented below. It is the opinion of the inventor that traffic is a difficult domain. Most traffic systems (all that the inventor knows about except the inventor""s) have fundamental shortages. The general shortage is that the systems are not considering the real-time and network characteristics of the traffic. The consequences will be that the systems would not operate in the way the system originators apparently have thought. The systems will not give any significant positive traffic function, but might even worsen the traffic situation.
The problem with the DRG-concept is related to the real-time requirement and the network characteristics. The central system sends the traffic information. Each vehicle, independent of each other, selects it route based on this information. Nobody knows what the summed traffic result will be. The central system doesn""t know where the cars are, which routes they select and then cannot foresee or prevent that traffic problems arise, e g because too many cars are arriving in the same period to an intersection. If the central system were equipped with sensors on the road network, those would after a while measure the traffic effects from the individually chosen vehicle routes. Then traffic problems might already have arisen or are arising. When at last the central system got that information, then it can send new traffic information including those detected problems, whereby each Neq will choose its new route. Those having possibility, will probably avoid the known problem area. The new routes however aren""t known or coordinated either, why problems now will occur on other places. The real-time requirement is put aside, as the DRG-system is too slow relative the traffic application, which is what the system should handle.
The network characteristics of traffic are put aside, as individual vehicles makes free route-selections, instead of a system performing route control based on coordination of traffic flows in a network.
In a known implementation of DRG (Socrates) the traffic feed back to the central system is not by road-based sensors, but of the vehicles themselves by sending messages telling the time elapsed in travelling a passed stretch. That means still longer time delay of the feed back, e g significant queues must have had time to grow before a vehicle later on can deliver information, which is proving a changed traffic situation.
The problems of the Lisb-concept are also found at another level. There it is required that in principal all vehicles must have the equipment and be connected to and operating in the system. Otherwise the central unit will not get the knowledge about traffic as it needs. That means that the system cannot be implemented in a small scale and be expanded successively. Also if more than half of the cars were included, which is an enormous amount in a large city, the unawareness about the routes of the others and their influence on the traffic flows would make route control meaningless of the known vehicles. Sure it is often only a few percent of the average flow on a first link that needs to be rerouted to another route, to get the average flow on the first flow to be below the capacity value of the link. So it is not the amount of controllable vehicles that is the problem, but the knowledge that is too small about the size of the traffic flows. The real-time requirements cause system difficulties also if all vehicles were in operation in the system. Let us assume that the system really would be able to give all vehicles optimal routes at the time t=t1. Those vehicles are spread over the whole network. During the next minute a large amount of new vehicles are starting their travel. They will arrive in intersections in time periods that already have been optimized for those starting earlier, t less than t1, from more distant sources in the network, and so on for the next minute etc. Further more during those one to two minutes many things have happened during the travel of the vehicles through the network. Certain vehicles haven""t arrived to an intersection in time for passage during the planned green period, but have to wait for the next, which might imply that a few seconds delay turns to 1, 5 minutes etc. That implies that for a period of a few minutes the whole system with lots of vehicles and their individual routes must be recalculated and optimized again for the new situation. Vehicles get new routes, which in the new perspective means that some of them wouldn""t have driven the route they really did. Their real route is no longer optimal. Optimization for a new situation means that the historical routes no longer have their former optimality. The original route control can also be that mismatched to the new changed circumstances, that implies traffic collapses and queues which are difficult to correct. There also might have arrived too many cars in an area, causing blocking each other, and cars might be route directed, travelling criss-cross through the network, and absorbing more link capacity than what is effective.
When more links in an area of the network are heavily loaded, i e links are congested or at the limit of being congested, there is little space left for guidance of arriving traffic. Wherever the excess traffic is directed, traffic collapses and queues will arise, which makes the system sensitive. Different kinds of problems might occur, e g those mentioned above, that the system tend to guide cars criss-cross in the network.
The real-time problem is mainly due to the system trying to control the routes of all cars from their origins to their destinations. In a large city travel times of 15 minutes to an hour are usual. That is a long prediction horizon, for control of e g a passage through an intersection, during the short intersection green period. During half an hour many things have time to happen in traffic in a heavily loaded network.
In spite of the large resources needed as equipment in all cars and perpetual comprehensive optimizing calculations, the system concept suffers from uncertainties and shortages, which anyhow might get the system to collapse and traffic to be blocked in the similar way as happens without the system. The concept is not automatically offering a solution on the mentioned fundamental traffic problems. Another solution is needed. The present invention is such a solution.
The present invention concerning route control is based on a new view on traffic and traffic problems, where real-time and network characteristics are distinctive features of a traffic management system""s possibility to really manage managing the traffic. Background material is the Swedish patents and applications: 9203474-3, 9501919-6, 9602950-9 and 9800280-1. The first is about traffic predictions in a network, the second about detecting traffic disturbances, the third about traffic management on motorways, the fourth about traffic management on a network. The content of those papers is regarded known and described methods can be used together with the present invention in a traffic management system for control of traffic. The fourth paper contains much of the problem view and background also for the present invention. So that information is not repeated here. The fourth paper contains relatively more general methods for traffic control in a network, while the present invention is focused on route management, and therefore specially created methods.
The method for route control of traffic on a road network is treating uncertainties in traffic. Uncertainties which are fundamentally inherent in the real-time and network characteristics of traffic. Uncertainties which the method handles by control of traffic margins. At rerouting a part of the traffic from a first route to a second route, one should know, if the extra traffic gets space on the second route. Uncertainties imply a need for margins to handle different kinds of deviations. If the flow, in spite of a low probability, grows too large, there is a need for margins within the traffic management, creating an ability to handle the situation without difficult consequences for the traffic. With storage spaces at the road network vehicles dynamically can be stored and released, e g the inflow into a link can exceed the outflow during a period. The margins for the route control in the management system are not only given by the difference between the capacity value and the current flow of the link. The total margin is given by the action possibilities of the management system. That includes actions as control of traffic margins and the flows on upstream links and nodes. Route control can also be performed at different hierarchic levels on the network:
Local level, concerning a local bottleneck.
Intermediate level, concerning a longer distance along a traffic route or a sub-area of the network.
Upper level, concerning route control between larger traffic routes or between different sub-areas in the network.